In the manufacture and testing of integrated circuits, it is necessary to use electrical probes to contact the circuit for various reasons including characterization of features and failure analysis. However, while many integrated circuits presently have sub-micron features, state of the art probe heads are limited to feature sizes of one micron or greater. This is due to the fact that both the optical microscopes used in conventional probe arrangements as well as the micrometer screws which are used to position the conventional probe tips are so limited.
Scanning proximity microscopes are known in the prior art, and are capable of imaging the surface of a substrate with atomic resolution. The scanning tunneling microscope is based on the principle that when two conductors are placed very close to each other, and a potential difference is applied therebetween, the electron clouds of the atoms on the respective surfaces closest to each other will touch and a tunnel current flows across the gap. In a scanning tunneling microscope, a conductive tip is scanned across a substrate, and since the magnitude of the tunneling current is dependent on the distance between the tip and the substrate, a correction signal is generated and is used to control the tunneling distance. The correction signal is plotted against physical position to provide the location of features.
A disadvantage of the scanning tunneling microscope is that both the tunnel tip and the surface being inspected must be made of conductive material. The atomic force microscope does not have this limitation, and is comprised of a small spring having a pointed tip which when brought very close to a substrate will be slightly deflected by the interatomic forces which occur between the two bodies, the magnitude of which is dependent on the distance therebetween. The deflection of the spring is measured by a tunnel tip which is spaced a small distance from the spring, a correction signal being developed as described above, and being plotted against physical position on the substrate. An atomic force microscope of this type is disclosed in Binnig U.S. Pat. No. 4,724,318, which is incorporated herein by reference.
While the atomic force microscope provides signals which correspond to the height of a substrate, there is no effective way in the prior art in which particular sub-micron features can be contacted by an electrical probe.